Colorimetric Detection of Nucleic Acids through Triplex-Hybridization Chain Reaction and DNA-Controlled Growth of Platinum Nanoparticles on Graphene Oxide

Ultrathin-FeOOH-coated MnO2 nanozyme with enhanced catalase-like and oxidase-like activities for photoelectrochemical and colorimetric detection of organophosphorus pesticides

Herein, we develop a dual-mode biosensor for photoelectrochemical and colorimetric detection of organophosphate pesticides (OPPs) based on ultrathin-FeOOH-coated MnO2 (MO@FHO) nanozyme. In this biosensor, OPPs can inhibit the alkaline phosphatase (ALP) activity and hinder the dephosphorylation of l-ascorbic acid-2-phosphate, preventing the decomposition of MO@FHO nanozyme and inducing both a photoelectrochemical (PEC) signal and the colorimetric change. The MO@FHO nanozyme not only possesses an enhanced catalase-like activity to degrade H2O2 for the generation of an improved cathodic photocurrent, but also exhibits an excellent oxidase-like activity to oxidize 3,3,5,5-tetramethylbenzidine with high catalytic efficiency. This biosensor displays a detection limit of 50 pmol/L for the PEC mode and a detection limit of 0.8 nmol/L for the colorimetric mode. Moreover, this biosensor exhibits excellent performance in complex biological matrices, and the smartphone-based visual sensing platform facilitates rapid and sensitive detection of OPPs, holding promising applications in food safety monitoring, and on-site detection.

Magnetic beads and GO-assisted enzyme-free signal amplification fluorescent biosensors for disease diagnosis

Cancer detection is still a major challenge in public health. Identification of oncogene is the first step toward solving this problem. Studies have revealed that various cancers are associated with miRNA expression. Therefore, the sensitive detection of miRNA is substantially important to solve the cancer problem. In this study, let-7a, a representative substance of miRNA, was selected as the detection target. With the assistance of magnetic beads commonly used in biosensors and self-synthesized graphene oxide materials, specificity and sensitivity detection of the target gene let-7a were achieved via protease-free signal amplification. The limit of detection (LOD) was as low as 15.015pM. The fluorescence signal intensity showed a good linear relationship with the logarithm of let-7a concentration. The biosensor could also detect let-7a in complex human serum samples. Overall, this fluorescent biosensor is not only simple to operate, but also strongly specificity to detect let-7a. Therefore, it has substantial potential for application in the early diagnosis of clinical medicine and biological research.

Multifunctional nanozymes for disease diagnosis and therapy

The development of nanotechnology has brought about groundbreaking advancements in diseases' diagnostics and therapeutics. Among them, multifunctional nanomaterials with enzyme-like activities (i.e., nanozymes) featured with high stability, large surface area for bioconjugation, and easy storage, offer unprecedented opportunities for disease diagnostics and treatment. Recent years have witnessed the great progress of nanozyme-based theranostics. To highlight these achievements, this review first introduces the recent advancements on nanozymes in biosensing and diagnostics. Then, it summarizes the applications of nanozymes in therapeutics including anti-tumor and antibacterial treatment, anti-inflammatory treatment, and other diseases treatment. In addition, several targeted strategies to improve the therapeutic efficacy of nanozyme are discussed. Finally, the opportunities and challenges in the field of diagnosis and therapy are summarized.

Polyvalent aptamer scaffold coordinating light-responsive oxidase-like nanozyme for sensitive detection of zearalenone

An efficient aptasensor was developed for the colorimetric determination of zearalenone (ZEN) based on polyvalent aptamer scaffold and light-responsive oxidase-like nanozyme. The sensitivity and efficiency of the development method were significantly improved owing to rich aptamers and signal labels (3, 4-dihydroxybenzoic acid, PCA) packed in the scaffold. The scaffold integrated functions of target recognition, surface immobilization and signal transduction. The photoresponsive nanoenzyme of TiO2-PCA was formed by PCA coordinated with Ti (IV) on the surface of TiO2. TiO2-PCA catalyzed dissolved oxygen rather than H2O2 to generate colorimetric signal by stimulating the chromogenic substrate, which made the assay greener and safer. The detection limit of colorimetric mode was 0.0087 ng/mL and the satisfactory recoveries 92.00 %-111.00 % were achieved in spiked food samples. This strategy opens new horizons for sensitive detection of small molecule hazards and promises to be a powerful tool for safeguarding food safety.

Aptamer tuned nanozyme activity of nickel-metal-organic framework for sensitive electrochemical aptasensing of tetracycline residue

It is urgently needed to develop high-performance method for tetracycline (TC) analysis to meet the growing concerns about food safety. Herein, a MOF of Ni2+-2,3,6,7,10,11-hexahydroxytriphenylene (Ni-HHTP) with peroxidase activity has been prepared and coated on a screen printing electrode, followed by non-covalent adsorption of tetracycline aptamer (TC-Apt) through the π-stacking. The spectroscopic and electrochemical assays show that TC-Apt can effectively enhance the nanozyme activity of Ni-HHTP using 3,3',5,5'-tetramethylbenzidine (TMB)/H2O2 as the probe. Upon binding with TC, the configuration of TC-Apt is changed and desorbs from the Ni-HHTP, resulting in the decrease of the nanozyme activity of aptasensor. Based on this principle, the target TC can be analyzed in concentrations ranging from 10 pM to 1.0 μM, with a detection limit of 1.9 pM. The aptsensor is also applicable for TC analysis in fresh Ctenopharyngodon idella meat and milk, which provides a new approach for TC residue monitoring in food.

Nanozymes: A comprehensive review on emerging applications in cancer diagnosis and therapeutics

Nanozymes, a new class of nanomaterials-based artificial enzymes, have gained huge attraction due to their high operational stability, working efficiency in extreme conditions, and resistance towards protease digestion. Nowadays, they are effectively substituted for natural enzymes for catalysis by closely resembling the active sites found in natural enzymes. Nanozymes can compensate for natural enzymes' drawbacks, such as high cost, poor stability, low yield, and storage challenges. Due to their transforming nature, nanozymes are of utmost importance in the detection and treatment of cancer. They enable precise cancer detection, tailored drug delivery, and catalytic therapy. Through enhanced diagnosis, personalized therapies, and reduced side effects, their adaptability and biocompatibility can transform the management of cancer. The review focuses on metal and metal oxide-based nanozymes, highlighting their catalytic processes, and their applications in the prevention and treatment of cancer. It emphasizes their potential to alter diagnosis and therapy, particularly when it comes to controlling reactive oxygen species (ROS). The article reveals the game-changing importance of nanozymes in the future of cancer care and describes future research objectives, making it a useful resource for researchers, and scientists. Lastly, outlooks for future perspective areas in this rapidly emerging field have been provided in detail.

Amplification-free microRNA profiling with femtomolar sensitivity on a plasmonic enhanced fluorescence nano-chip

MicroRNAs (miRNAs) are a class of small, non-coding RNA molecules involved in the regulation of gene expression, thus considered as promising biomarkers for cancer, cardiovascular diseases, neurodegenerative diseases, etc. However, quantitative analysis of miRNAs faces challenges owing to their high homology, small size & ultra-low abundance, and disease occurrence is often related to abnormal expression of multiple miRNAs where method for parallel miRNAs analysis is required. In this work, multiplexed analysis of miRNAs was established on a plasmonic nano-chip capable of fluorescence enhancement in the near-infrared region. Combined with polyadenylation at the hydroxyl terminate of target miRNA to afford abundant sites for fluorophore labeling, our assay achieved amplification-free detection of miRNAs from nM to fM with the limit of detection down to ca. 5 fM. A miRNA panel was constructed to detect 10 miRNAs differentially expressed in MCF-7 and A549 cell lines and validated with qRT-PCR, demonstrating the practical application of this method. This scalable platform can be customized for different miRNA panels, facilitating multiple miRNA profiling for various diseases.

Signal Amplification Strategy Design in Nanozyme-Based Biosensors for Highly Sensitive Detection of Trace Biomarkers

Nanozymes show great promise in enhancing disease biomarker sensing by leveraging their physicochemical properties and enzymatic activities. These qualities facilitate signal amplification and matrix effects reduction, thus boosting biomarker sensing performance. In this review, recent studies from the last five years, concentrating on disease biomarker detection improvement through nanozyme-based biosensing are examined. This enhancement primarily involves the modulations of the size, morphology, doping, modification, electromagnetic mechanisms, electron conduction efficiency, and surface plasmon resonance effects of nanozymes for increased sensitivity. In addition, a comprehensive description of the synthesis and tuning strategies employed for nanozymes has been provided. This includes a detailed elucidation of their catalytic mechanisms in alignment with the fundamental principles of enhanced sensing technology, accompanied by the presentation of quantitatively analyzed results. Moreover, the diverse applications of nanozymes in strip sensing, colorimetric sensing, electrochemical sensing, and surface-enhanced Raman scattering have been outlined. Additionally, the limitations, challenges, and corresponding recommendations concerning the application of nanozymes in biosensing have been summarized. Furthermore, insights have been offered into the future development and outlook of nanozymes for biosensing. This review aims to serve not only as a reference for enhancing the sensitivity of nanozyme-based biosensors but also as a catalyst for exploring nanozyme properties and their broader applications in biosensing.

Highly sensitive detection of microRNA-21 by nitrogen-doped carbon dots-based ratio fluorescent probe via nuclease-assisted rolling circle amplification strategy

Highly sensitive and selective detection of microRNA-21 (miRNA-21) in biological samples is critical for the disease diagnosis and cancer treatment. In this study, a nitrogen-doped carbon dots (N-CDs)-based ratio fluorescence sensing strategy was constructed for miRNA-21 detection with high sensitivity and excellent specificity. Bright-blue N-CDs (λ_ex/λ_em = 378 nm/460 nm) were synthesized by facile one-step microwave-assisted pyrolysis method by using uric acid as the single precursor, and the absolute fluorescence quantum yield and fluorescence lifetime of N-CDs were 35.8% and 5.54 ns separately. The padlock probe hybridized with miRNA-21 firstly and then was cyclized by T4 RNA ligase 2 to form a circular template. At the present of dNTPs and phi29 DNA polymerase, the oligonucleotide sequence in miRNA-21 was prolonged to hybridize with the surplus oligonucleotide sequences in circular template, generating long and reduplicated oligonucleotide sequences containing abundant guanine nucleotides. Separate G-quadruplex sequences were generated after the addition of Nt.BbvCI nicking endonuclease, and then hemin bound with G-quadruplex sequence to construct the G-quadruplex DNAzyme. Such G-quadruplex DNAzyme catalyzed the redox reaction of o-phenylenediamine (OPD) with H₂O₂, finally producing the yellowish-brown 2,3-diaminophenazine (DAP) (λ_em = 562 nm). Due to the inner filter effect between N-CDs and DAP, the ratio fluorescence signal of DAP with N-CDs was utilized for sensitive detection of miRNA-21 with detection limit of 0.87 pM. Such approach has practical feasibility and excellent specificity for miRNA-21 analysis during highly homological miRNA family in HeLa cell lysates and human serum samples.

A triple-cycle amplification biosensor for colorimetric detection of mutant PIK3CAE545K based on cascade-driven DNA walker and branched hybridization strand reaction

Circulating tumor DNA (ctDNA) is an ideal candidate for liquid biopsy biomarkers. Therefore, detecting a low abundance of ctDNA is essential for early cancer diagnosis. Here, we developed a novel triple circulation amplification system integrating entropy and enzyme cascade-driven three-dimensional (3D) DNA walker and branched hybridization strand reaction (B-HCR) for ultrasensitive detection of breast cancer-related ctDNA. In this study, the 3D DNA walker was constructed by inner track probes (NH) and complex S on a microsphere. Once the DNA walker was triggered by the target, the strand replacement reaction ran first and kept circulating to rapidly displace the DNA walker containing 8-17 DNAzyme. Secondly, the DNA walker could repeatedly cleave NH autonomously along the inner track, generating numerous initiators, and then promoting B-HCR to activate the third cycle. Subsequently, the split G-rich fragments were brought in close to form the G-quadruplex/hemin DNAzyme by adding hemin, with the addition of H₂O₂ and ABTS, the target could be observed. Benefiting from triplex cycles, the PIK3CA^E545K mutation detection possesses a good linear range from 1-10³ fM, and the limit of detection was 0.65 fM. Due to the low cost and high sensitivity, the proposed strategy has great potential in early diagnosis of breast cancer.

Entropy-driven DNA circuit with two-stage strand displacement for elegant and robust detection of miRNA let-7a

MicroRNAs (miRNAs) research in cancer diagnosis is expanding, on account of miRNAs were demonstrated to be key indicator of gene expression and hopeful candidates for biomarkers. In this study, a stable miRNA-let-7a fluorescent biosensor was successfully designed based on an exonuclease Ⅲ-assisted two-stage strand displacement reaction (SDR). First, an entropy-driven SDR containing a three-chain structure of the substrate is used in our designed biosensor, leading to reduce the reversibility of the target recycling process in each step. The target acts on the first stage to start the entropy-driven SDR, which generates the trigger used to stimulate the exonuclease Ⅲ-assisted SDR in the second stage. At the same time, we design a SDR one-step amplification strategy as a comparison. Expectly, this developed two-stage strand displacement system has a low detection limit of 25.0 pM as well as a broad detection range of 4 orders of magnitude, making it more sensitive than the SDR one-step sensor, whose detection limit is 0.8 nM. In addition, this sensor has high specificity across members of the miRNA family. Therefore, we can take advantage of this biosensor to promote miRNA research in cancer diagnosis sensing systems.

Visual and Electrochemical Detection of let-7a: A Tumor Suppressor and Biomarker

Let-7a, a type of low-expressed microRNAs in cancer cells, has been investigated as a promising biomarker and therapeutic target for tumor suppression. Developing simple and sensitive detection methods for let-7a is important for cancer diagnosis and treatment. In this work, the hybridization chain reaction (HCR) was initiated by let-7a via two hairpin primers (H1 and H2). After the HCR, the remaining hairpin H1 was further detected by lateral flow assay (LFA) and electrochemical impedance spectroscopy. For LFA, biotin-modified H1(bio-H1) and free H2 were used for HCR. With the decrease of let-7a concentration, the color of T line gradually increased. As for electrochemical methods, the H1'-AuNP-modified electrode was used for detection of bio-H1 based on the difference of impedance (ΔR_ct) detected without and with different concentrations of let-7a participating in the HCR. This method could detect let-7a in the range of 10.0 fM and 1.0 nM with detection limits of 4.2 fM.

N-Doped Carbon Nanotubes Supported Fe-Mn Dual-Single-Atoms Nanozyme with Synergistically Enhanced Peroxidase Activity for Sensitive Colorimetric Detection of Acetylcholinesterase and Its Inhibitor

Monitoring acetylcholinesterase (AChE) and its inhibitors is of importance for early diagnosis and therapy of neurological diseases. Herein, N-doped carbon nanotubes supported Fe-Mn dual-single-atoms (FeMn DSAs/N-CNTs) were fabricated by a simple pyrolysis, as thoroughly figured out by a series of the characterization techniques. The peroxidase-like activity of FeMn DSAs/N-CNTs was investigated by catalytic oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to generate rich hydroxyl radicals (·OH) in the H₂O₂ system, which effectively catalyzed colorless TMB oxidation to blue oxidized TMB (ox-TMB). Besides, the peroxidase-like activity was greatly weakened by thiocholine (derived from AChE), accompanied by making blue ox-TMB fade. Impressively, the highly improved peroxidase-like property is further evidenced by density functional theory (DFT) calculations, where the dual-single atoms show a lower energy barrier (0.079 eV) and their interactions with the N-CNTs played critical roles for producing the oxygen radicals. By virtue of the nanozyme, a low-cost, specific, and sensitive colorimetric sensor was built for detection of AChE with a broader linear range of 0.1-30 U L^-1 and a lower limit of detection (LOD, 0.066 U L^-1), combined with its feasible analysis in human serum samples. Also, this platform was applied for measuring huperzine A inhibitor with a wide linear scope of 5-500 nM and a LOD down to 4.17 nM. This strategy provides a low-cost and convenient approach for early clinical diagnosis and drug development.

Ruthenium-Encapsulated Porphyrinic Organic Polymer as a Photoresponsive Oxidoreductase Mimetic Nanozyme for Colorimetric Sensing

The advantages of porosity and stable unpaired electrons of porphyrinic organic polymers (POPs) with free radicals are exclusive and potentially practical functionalities and combining the semiconductor-like characteristics of these materials and metal ions has been an effective way to assemble an efficient photocatalytic system. Herein, a new ruthenium (Ru) ion-encapsulated porphyrinic organic polymer (POP/Ru) is facilely synthesized as a proper photoresponsive nanozyme with unique photo-oxidase properties. Surprisingly, the proposed POP/Ru revealed outstanding photoresponsive oxidase-mimicking activity due to the synergetic effect of the integration of Ru and π-electrons of POP, which boosts charge separation and transport. POP/Ru was applied to the oxidation of o-phenylenediamine (o-PDA) as a chromogenic probe for producing a colorimetric signal. The kinetic study reveals that these photo-oxidase mimics have a significant affinity for the o-PDA chromogenic agent owing to a lower K_m and superior V_max. Further findings demonstrate that the presence of the l-arginine (l-Arg) target causes an inhibition effect on the photo-nanozymatic colorimetry of POP/Ru. This research develops the applications of the comprehensive colorimetric strategy for ultrasensitive l-Arg monitoring with a limit of detection (LOD) of 15.2 nM in the dynamic range of 4.0 nM-340 μM and illuminates that the proposed photo-oxidase nanozyme as a visual strategy is feasible in l-Arg environmentally friendly colorimetric detection in juice samples.

Photo-responsive oxidase-like nanozyme based on a vanadium-docked porphyrinic covalent organic framework for colorimetric L-Arginine sensing

This study reports the development of a vanadium-docked porphyrinic covalent organic framework as a novel class of highly polar photoactive materials. Thanks to its extended π-electron conjugation and high chemical stabilities, this framework can serve as an oxidase-Like photo-nanozyme for photocatalytic oxidation of o-phenylenediamine (o-PDA) and a colorimetric substrate for the production of the yellow-colored oxidized o-PDA (o-PDAox). The physicochemical properties of the as-prepared photo-nanozyme were characterized by several analytical techniques. Its enhanced light harvesting and charge separation and transfer were also verified by electrochemical and spectroscopic analysis. This photo-nonenzymatic colorimetric assay was applied for the sensitive L-Arginine (L-Arg) detection as a typical amino acid in the linear range of 8.1 nM-330 μM with a limit of detection (LOD) of 3.5 nM. The findings of this research confirmed the safety and feasibility of the proposed photo-nonenzymatic colorimetric sensing strategy for the detection of L-Arg and other similar biomolecules in food samples. Kinetic investigation revealed that the photo-responsive oxidase mimic exhibits satisfactory K_m (0.47 mM) and V_max (42.0 μM/s) values. This work broadened our insight into the development of modified porphyrinic-COF-based visible light-responsive oxidase-like photo-nanozyme for environmentally friendly colorimetric biosensing.

Development of a hybridization chain reaction-powered lab-on-fiber device for ultrafast point-of-care testing of circulating tuor DNA in whole blood

Circulating tumor DNA (ctDNA) demonstrates great promise in the guidance of prognostication, diagnosis, and surveillance of cancers, which highlights the need for rapid and sensitive point-of-care testing (POCT) technologies. Hybridization chain reaction (HCR)-based optical biosensors provide excellent solutions due to their prominent features. However, the requirement of a sophisticated and expensive optical readout device, relatively long detection time, and heating hold back their scalability and clinical applications. Here, an innovative HCR-powered lab-on-fiber device (HCR-LOFD) was developed for rapid on-site detection of ctDNA with high sensitivity, specificity, and reproducibility. A LOFD with a compact all-fiber optical structure was constructed for the fluorescence detection of the HCR system. Combining HCR, fluorescence energy resonant transfer, and the evanescent wave fluorescence principle, HCR-LOFD achieved the quantitative detection of KRAS G12D, the 12th amino acid from glycine (Gly) mutated aspartate (Asp) and the most common mutation of KARS, in 5 min at room temperature based on end-point detection mode or real-time fluorescence detection mode. This new assay platform was also successfully applied for the direct detection of KRAS G12D in whole blood with simple dilution. The application of HCR-LOFD not only greatly simplifies the complexity of optical readout devices and improves their scalability but also potentially serves as a sample-to-answer solution for the detection of biomarkers in limited medical resource regions.

One-step synthesis of triethanolamine-capped Pt nanoparticle for colorimetric and electrochemiluminescent immunoassay of SARS-CoV spike proteins

Platinum nanoparticles (PtNPs) have been attracted worldwide attention due to their versatile application potentials, especially in the catalyst and sensing fields. Herein, a facile synthetic method of triethanolamine (TEOA)-capped PtNPs (TEOA@PtNP) for electrochemiluminescent (ECL) and colorimetric immunoassay of SARS-CoV spike proteins (SARS-CoV S-protein, a target detection model) is developed. Monodisperse PtNPs with an average diameter of 2.2 nm are prepared by a one-step hydrothermal synthesis method using TEOA as a green reductant and stabilizer. TEOA@PtNPs can be used as a nanocarrier to combine with antigen by the high-affinity antibody, which leads to a remarkable inhibition of electron transfer efficiency and mass transfer processes. On the basis of its peroxidase-like activity and easy-biolabeling property, the TEOA@PtNP can be used to establish a colorimetric immunosensor of SARS-CoV S-protein thought catalyzing the reaction of H₂O₂ and 3,3',5,5'-tetramethylbenzidine (TMB). Especially, the Ru(bpy)₃ ²⁺ ECL reaction is well-achieved with the TEOA@PtNPs due to their great conductivity and loading abundant TEOA co-reactants, resulting in an enhancing ECL signal in immunoassay of SARS-CoV S-protein. As a consequence, two proposed methods could achieve sensitive detection of SARS-CoV S-protein in wide ranges, the colorimetric and ECL detection limits were as low as 8.9 fg /mL and 4.2 fg /mL (S/N = 3), respectively. We believe that the proposed colorimetric and ECL immunosesors with high sensitivity, good reproducibility, and good stability will be a promising candidate for a broad spectrum of applications.

Label-free microRNA detection using a locked-to-unlocked transforming system assembled by microfluidics

MicroRNA (miRNA) is a potential biomarker for the early screening and diagnosis of cancers and is widely present in human blood, urine and saliva. Here, we report a microfluidics-assembled tool for miRNA detection based on the regulation of DNA locked and unlocked states and explore its application in complex samples. Microfluidic techniques are used to continuously assemble the locked-to-unlocked transforming system using a rapid one-step method. It only takes 2 min to produce enough locked-to-unlocked systems for a miRNA detection experiment. DNA molecules with a recognition sequence and a G-rich reporter sequence (G4m) are locked by attaching both ends to the surface of magnetic beads (MBs) in microchannels. The presence of the target miRNA can initiate the specific cleavage of one end of G4m by duplex-specific nuclease, resulting in the transition of G4m from a locked state to an unlocked state. This transition enables G4m to freely fold into a G-quadruplex, which can participate in the catalysis of ABTS oxidation and result in a turquoise color. During the whole process, the target miRNA remains intact and continuously initiate specific cleavage, facilitating signal amplification. Magnetic separation steps are employed to assist in miRNA enrichment and interference reduction. As a proof of concept, we quantified miRNA-21 using the locked-to-unlocked system. The assay allows specific detection of miRNA-21 in the range of 3.2-570 pM with a detection limit of 2.01 pM (S/N = 3). Furthermore, the locked-to-unlocked system is used to analyze miRNA-spiked urine, saliva and serum samples and shows robust performance in different matrices.

Nanomaterial-Based Fluorescent Biosensor for Food Safety Analysis

Food safety issues have become a major threat to public health and have garnered considerable attention. Rapid and effective detection methods are crucial for ensuring food safety. Recently, nanostructured fluorescent materials have shown considerable potential for monitoring the quality and safety of food because of their fascinating optical characteristics at the nanoscale. In this review, we first introduce biomaterials and nanomaterials for food safety analysis. Subsequently, we perform a comprehensive analysis of food safety using fluorescent biosensors based on nanomaterials, including mycotoxins, heavy metals, antibiotics, pesticide residues, foodborne pathogens, and illegal additives. Finally, we provide new insights and discuss future approaches for the development of food safety detection, with the aim of improving fluorescence detection methods for the practical application of nanomaterials to ensure food safety and protect human health.

Click Chemistry Actuated Exponential Amplification Reaction Assisted CRISPR-Cas12a for the Electrochemical Detection of MicroRNAs

MicroRNAs (miRNAs) play a very important role in biological processes and are used as biomarkers for the detection of a variety of diseases, including neurodegenerative diseases, chronic cardiovascular diseases, and cancers. A sensitive point-of-care (POC) method is crucial for detecting miRNAs. Herein, CRISPR-Cas12a combined with the click chemistry actuated exponential amplification reaction was introduced into an electrochemical biosensor for detecting miRNA-21. The target miRNA-21 initiated the click chemistry-exponential amplification reaction in the electrochemical biosensor to produce numerous nucleic acid fragments, which could stimulate the trans-cleavage ability of CRISPR-Cas12a to cleave hairpin DNA electrochemical reporters immobilized on the electrode surface. Under optimal conditions, the minimum detection limit for this electrochemical biosensor was as low as 1 fM. Thus, the proposed electrochemical biosensor allows sensitive and efficient miRNA detection and could be a potential analysis tool for POC test and field molecular diagnostics.

Decorating Zirconium on Graphene Oxide to Design a Multifunctional Nanozyme for Eco-Friendly Detection of Hydrogen Peroxide

Peroxidase enzymes are crucial in analytical chemistry owing to significant peroxide analytes and their key role in hydrogen peroxide (H2O2) detection. Therefore, exploiting appropriate catalysts for the peroxidase like reactions has become crucial for achieving desired analytical performance. Zirconium (Zr) has attracted growing interest, as a safe and stable potential eco-friendly catalyst for various organic transformations that address increasing environmental challenges. Hence, aiming at fast, sensitive and selective optical detection of H2O2, a colorimetric platform is presented here, based on the excellent peroxidase enzyme-like activity of Zr decorated on graphene oxide (GO). The synergistic effect achieved due to intimate contact between an enzyme like Zr and the high surface area 0f GO ensures efficient electron transfer that increases the chemical and catalytic activity of the composite and advances the decomposition of H2O2 into hydroxyl radicals. The designed probe, thus, efficiently catalyzes the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB), via hydroxyl radicals, thereby transforming the colorless TMB into blue oxidized TMB within 2 min. The catalytic mechanism of the Zr-GO enzyme mimic is proposed herein and verified using a fluorescent probe terephthalic acid (TA) and other scavenger experiments. The multifunctional optical probe allows sensitive and highly selective recognition of H2O2 in a linear range from 100 to 1000 µM with a low detection limit of 0.57 µM. Essentially, the direct accessibility of Zr prevents having to use the complicated preparation and purification procedures mostly practiced for conventional biozymes and nanozymes. The devised method offers several gains, including being green and an inexpensive catalyst, having lower LOD, being fast, cost-effective and sensitive, and having selective work-up procedures.

Versatile Electrochemiluminescence Biosensing Platform Based on DNA Nanostructures and Catalytic Hairpin Assembly Signal Amplification

Achieving rapid and highly sensitive detection of biomarkers is crucial for disease diagnosis and treatment. Here, a highly sensitive and versatile dual-amplification electrochemiluminescence (ECL) biosensing platform was constructed for target detection based on DNA nanostructures and catalyzed hairpin assembly (CHA). Specifically, when the target DNA was present, it would hybridize with the auxiliary strands (D1 and D2) to form an I-shaped nanostructure, which in turn triggered the subsequent catalytic hairpin assembly reaction to generate plenty of double-stranded DNA complexes (H1-H2). The resulting double-stranded complex could be trapped on the electrode surface and adsorbed the ECL signal probe Ru(phen)₃²⁺.We found that the I-shaped nanostructure-triggered CHA reaction had higher amplification efficiency compared with traditional CHA amplification. Thus, a sensitive "signal-on" ECL biosensor was constructed for target DNA detection with a detection limit of 1.09 fM. Additionally, by combining the binding properties of C-Ag⁺-C with an elaborately designed "Ag⁺-helper" probe, the proposed strategy could be immediately utilized for the highly sensitive and selective detection of silver ions, demonstrating the versatility of the developed biosensing platform. This strategy provided a new approach with potential applications in disease diagnosis and environmental monitoring.

Highly Efficient Electrochemiluminescence of MnS:CdS@ZnS Core-Shell Quantum Dots for Ultrasensitive Detection of MicroRNA

In this work, a novel electrochemiluminescence (ECL) biosensor was developed for ultrasensitive detection of microRNA let-7a (miRNA let-7a) based on MnS:CdS@ZnS core-shell quantum dots (QDs) as ECL luminophores with high ECL efficiency. Impressively, compared to the CdS:Mn@ZnS QDs prepared by ionic doping with ECL efficiency of 0.87%, MnS:CdS@ZnS QDs synthesized by bimetallic clusters (Cd₂Mn₂O₄) doping exhibited high ECL efficiency of up to 15.84% with S₂O₈^2- as cathodic coreactant due to the elimination of the dopants size mismatch and "self-purification" effect, which could achieve the surface defect passivation of MnS:CdS@ZnS QDs for effectively improving the ECL emission. Furthermore, with the help of strand displacement amplification (SDA), the trace target miRNA let-7a was able to be converted to a number of output DNA labeled with ferrocene (Fc) to construct an ultrasensitive ECL biosensor. The well-designed ECL biosensor for miRNA let-7a exhibited high stability and excellent sensitivity of a concentration variation from 10 aM to 1 nM and a low detection limit of 4.1 aM, which was further applied to the analysis of miRNA let-7a from cancer cell (MCF-7) lysate. Thus, this strategy provides a novel method to prepare high-efficient ECL emitters for the construction of ECL biosensing platforms in biological fields and clinical diagnosis.

Sensitive and convenient detection of miRNA-145 using a gold nanoparticle-HCR coupled system: computational and in vitro validations

Multiple sclerosis (MS) remains a challenging disease that requires timely diagnosis. Therefore, an ultrasensitive optical biosensor based on hybridization chain reaction (HCR) was developed to detect microRNA-145 (miRNA-145) as an MS biomarker. To construct such a sensor, HCR occurred between specific hairpin probes, as MB1 contains a poly-cytosine nucleotide loop and MB2 has a poly-guanine nucleotide sticky end. By introducing miR-145 as a target sequence, long-range dsDNA polymers are formed. Then, positively charged gold nanoparticles (AuNPs) were incubated with the HCR product, which adsorbed onto the dsDNA polymers due to electrostatic adsorption. This resulted in the precipitation of the AuNPs. By incubating different concentrations of miR-145 with AuNPs, the changes in the UV-vis spectrum of the supernatant were analyzed. The proposed biosensor showed a great ability to detect miR-145 in a wide linear range from 1 pM-1 nM with an excellent detection limit (LOD) of 0.519 nM. Furthermore, the developed biosensor indicated considerable selectivity in discriminating between miR-145 and mismatched sequences. It shows high selectivity in differentiating targets. Interestingly, the proposed method was also able to detect miRNA-145 in the diluted serum samples. In conclusion, this sensing platform exhibits high selectivity and specificity for the detection of circulating microRNAs, which holds great promise for translation to routine clinical applications.

Acetylcholinesterase Activity Monitoring and Natural Anti-neurological Disease Drug Screening via Rational Design of Deep Eutectic Solvents and CeO2-Co(OH)2 Nanosheets

The activity monitoring of acetylcholinesterase (AChE) and the screening of its inhibitors are critical for the diagnosis and therapy of neurological diseases. Herein, CeO₂-Co(OH)₂ nanosheets were synthesized for the first time in a newly designed deep eutectic solvent (DES) composed of l-proline and Ce(NO₃)₃·6H₂O, and a colorimetric assay was developed for quantitative detection of AChE and anti-neurological disease drug screening. Impressively, CeO₂-Co(OH)₂ composites prepared in DESs have more prominent oxidase-like activity than Co(OH)₂, CeO₂, and CeO₂-Co(OH)₂ produced in aqueous solution. The mechanism study shows that the oxygen vacancies of CeO₂-Co(OH)₂ play a vital role in oxidase-like catalysis. Based on their excellent oxidase-like activity, the CeO₂-Co(OH)₂ nanosheets have been successfully applied for highly sensitive and selective detection of AChE with a linear range of 0.2-20 mU/mL. This strategy can also be used for inhibitor screening. The sensor displays an excellent linear response in the range of 0.001-2 μg/mL toward an irreversible inhibitor (paraoxon-ethyl). Moreover, five alkaloids, namely, berberine hydrochloride, caffeine, camptothecin, matrine, and evodiamine, were screened by using neostigmine bromide as a control; berberine hydrochloride exhibited a good inhibitory effect on AChE with an IC₅₀ of 0.94 μM, while the other four had no obvious inhibitory effect. The mechanism of the different effects of alkaloids on inhibiting acetylcholinesterase activity was explored via molecular docking and kinetic simulation.

Ultrasensitive quantitation of circulating miR-195-5p with triple strand displacement amplification cascade

Circulating miR-195-5p has been proposed as a promising peripheral biomarker for the diagnosis, prognosis and severity assessment of various diseases. However, the demand for its sensitive and convenient quantification has not been met yet. Herein, we proposed a one-pot isothermal approach, in which the target signal acquisition, amplification and conversion (fluorescence read-out) system was integrated by a triple strand displacement amplification (SDA) cascade. Using this triple SDA strategy, miR-195-5p can be at least detected at 1 aM, and the linear dynamic range (from 100 aM to 1 pM) is wide enough to meet the detection needs of clinical miRNA level. A proof-of-principle study, using this novel methodology to directly analyze the spiking serum samples with different levels of miR-195-5p, demonstrated the potential of circulating miR-195-5p detection for clinical point-of-care assay. This one-pot isothermal triple SDA approach, we believe, will be a simple and feasible tool for ultrasensitive quantification of circulating miR-195-5p, and may promote the wide application of this potential biomarker in non-invasive clinical diagnosis.

Applications of hybridization chain reaction optical detection incorporating nanomaterials: A review

The development of powerful, simple and cost-effective signal amplifiers has significant implications for biological research and analysis. Hybridization chain reaction (HCR) has attracted increasing attention because of its enzyme-free, simple, and efficient amplification. In the HCR process, an initiator probe triggered a pair of metastable hairpins through a cross-opening process to propagate a chain reaction of hybridization events, yielding a long-nicked double-stranded nucleic acid structure. To achieve more noticeable signal amplification, nanomaterials, including graphene oxide, quantum dots, gold, silver, magnetic, and other nanoparticles, were integrated with HCR. Various types of colorimetric, fluorescence, plasmonic analyses or chemiluminescence optical sensing strategies incorporating nanomaterials have been developed to analyze various targets, such as nucleic acids, small biomolecules, proteins, and metal ions. This review summarized the recent advances of HCR technology pairing diverse nanomaterials in optical detection and discussed their challenges.

Engineering entropy-driven based multiple signal amplification strategy for visualized assay of miRNA by naked eye

MicroRNAs (miRNAs) are currently recognized as novel biomarkers for cancer early diagnosis, therapy selection, and progression monitoring. Herein, we developed an ultrasensitive and label-free homogeneous colorimetric strategy for miRNA detection based on engineering entropy-driven amplification (EDA) coupled with nicking enzyme-assisted AuNP aggregation. In our design, the target miRNA could specifically trigger the EDA recycling process. One of the EDA products could open the hairpin probe and form a dual strand containing a nicking endonuclease (Nb.BbvCl) cleavage region. After adding nicking endonuclease in the sensing solution, the product DNA fragments could act as two linkers, inducing the aggregation of ssDNA-modified AuNPs. Simultaneously, the liberating complementary strands continued to cyclic hybridization with the hairpin probe. This multiple signal amplification colorimetric strategy showed a wide linear range from 10 fM to 100 pM with a much lower detection limit of 3.13 fM for miRNA let-7a, which also performed well in a complex sample matrix. Most importantly, the naked eye could clearly distinguish the 10 fM color change caused by let-7a to be measured. Moreover, this approach could easily extend to multiple miRNAs with target-specific sequence substitutions. Therefore, this ultrasensitive visual strategy for miRNA demonstrated attractive potentials for promising applications in clinical diagnosis.

Strand Displacement Amplification Assisted CRISPR-Cas12a Strategy for Colorimetric Analysis of Viral Nucleic Acid

The development of a sensitive, facile, and cost-effective colorimetric method is of great significance for the point-of-care testing of viral nucleic acid. Herein, we reported a strand displacement amplification assisted CRISPR-Cas12a (SDACC) method for the colorimetric analysis of viral nucleic acid. The hepatitis B virus (HBV) DNA was chosen as the target to trigger strand displacement amplification (SDA) and generate abundant single-strand DNA (ssDNA) products. The ssDNA amplicon hybridized with template DNA to activate the trans-cleavage activity of CRISPR-Cas12a, leading to the nonspecific cleavage of ssDNA on GOx-ssDNA-modified magnetic beads and the release of GOx. The released GOx was capable of catalyzing the substrate solution to generate a color change, which could be directly observed by naked eyes. The SDACC strategy could identify a single-base mismatch located in the DNA sequence and achieve a sensitive detection for HBV DNA with the limit of detection as low as 41.8 fM. Notably, the sophisticated primer design for target amplification and complicated detection process could be circumvented. The current approach realizes a simple, low-cost, and sensitive colorimetric detection for viral nucleic acid and holds great promise for the practical application of virus infection diagnosis.

Photoelectrochemical Assay Based on SnO2/BiOBr p-n Heterojunction for Ultrasensitive DNA Detection

Herein, a photoelectrochemical (PEC) assay was designed for a highly sensitive DNA determination relying upon the SnO₂/BiOBr p-n heterojunction as a photoactive material and SiO₂ as a signal quencher. Compared with most traditional heterojunctions, the SnO₂/BiOBr p-n heterostructure not only lessened the recombination of the photogenerated electron-hole pairs but also promoted the light-harvesting in the ultraviolet-visible (UV-vis) region, leading to further enhanced photoelectric conversion efficiency and photocurrent, which demonstrated 12.1-fold and 6.4-fold increments versus those of pure SnO₂ and BiOBr, respectively. Additionally, the limited quantity of target DNA (a fragment of p53 gene) could be transformed into abundant output DNA-SiO₂ by employing the Nt·BstNBI enzyme-assisted signal amplification procedure, leading to a highly improved detection sensitivity of the biosensor. Then, output DNA-SiO₂ hybridized with the capture DNA anchored on the modified electrode surface, remarkably diminishing the PEC signal and thus achieving sensitive DNA determination. The elaborated PEC biosensor demonstrated outstanding performance within the linear range between 0.5 fM and 5 nM and a low limit of detection down to 0.18 fM, paving a new way for fabricating heterojunction with exceptional photoactive performance and demonstrating the enormous potential for detecting multitudinous biomarkers in bioanalysis and clinical therapy.

Rapid and highly sensitive hairpin structure-mediated colorimetric detection of miRNA

Herein, we report a novel hairpin structure-mediated diagnostic method for the simple and rapid colorimetric detection of miRNA through the sensing of pyrophosphate. When the hairpin structure of the template DNA (h-Probe) was hybridized with the primer, the DNA primer extension mediated by nPfu special enzyme was blocked. However, this h-Probe was extended using nPfu special enzyme, upon the structural change of the template DNA, from a hairpin to a linear structure, in the presence of the target miRNA. The miRNA-hybridized template DNA sequence was cleaved by a duplex-specific nuclease (DSN), which cleaved the DNA from the RNA-DNA hybrid, thereby allowing the target miRNA to be recycled. Primer extension using nPfu special enzyme produced pyrophosphate when nucleotide triphosphate was incorporated into the DNA; this pyrophosphate was sensed in terms of a color change, from pink to colorless, when using pp Probe, a probe developed previously by our group. This novel system for the colorimetric detection of target miRNA operated with high sensitivity (LOD = 132 aM) and selectivity, with the whole detection process requiring only 30 min. Furthermore, this system could also detect miRNA fluorimetrically with similar sensitivity (LOD = 105 aM), highlighting the dual-sensing properties of pp Probe. This unique, extremely simple, and rapid system for the detection of miRNA through a highly sensitive color change would presumably be useful in applications requiring point-of-care detection.

Recent Progress in DNA Hybridization Chain Reaction Strategies for Amplified Biosensing

With the continuous development of DNA nanotechnology, various spatial DNA structures and assembly techniques emerge. Hybridization chain reaction (HCR) is a typical example with exciting features and bright prospects in biosensing, which has been intensively investigated in the past decade. In this Spotlight on Applications, we summarize the assembly principles of conventional HCR and some novel forms of linear/nonlinear HCR. With advantages like great assembly kinetics, facile operation, and an enzyme-free and isothermal reaction, these strategies can be integrated with most mainstream reporters (e.g., fluorescence, electrochemistry, and colorimetry) for the ultrasensitive detection of abundant targets. Particularly, we select several representative studies to better illustrate the novel ideas and performances of HCR strategies. Theoretical and practical utilities are confirmed for a range of biosensing applications. In the end, a deep discussion is provided about the challenges and future tasks of this field.

Nanozymes-Hitting the Biosensing "Target"

Nanozymes are a class of artificial enzymes that have dimensions in the nanometer range and can be composed of simple metal and metal oxide nanoparticles, metal nanoclusters, dots (both quantum and carbon), nanotubes, nanowires, or multiple metal-organic frameworks (MOFs). They exhibit excellent catalytic activities with low cost, high operational robustness, and a stable shelf-life. More importantly, they are amenable to modifications that can change their surface structures and increase the range of their applications. There are three main classes of nanozymes including the peroxidase-like, the oxidase-like, and the antioxidant nanozymes. Each of these classes catalyzes a specific group of reactions. With the development of nanoscience and nanotechnology, the variety of applications for nanozymes in diverse fields has expanded dramatically, with the most popular applications in biosensing. Nanozyme-based novel biosensors have been designed to detect ions, small molecules, nucleic acids, proteins, and cancer cells. The current review focuses on the catalytic mechanism of nanozymes, their application in biosensing, and the identification of future directions for the field.

Programmable DNA Circuits for Flexible and Robust Exciton-Plasmon Interaction-Based Photoelectrochemical Biosensing

DNA circuits as one of the dynamic nanostructures can be rationally designed and show amazing geometrical complexity and nanoscale accuracy, which are becoming increasingly attractive for DNA entropy-driven amplifier design. Herein, a novel and elegant exciton-plasmon interaction (EPI)-based photoelectrochemical (PEC) biosensor was developed with the assistance of a programmable entropy-driven DNA amplifier and superparamagnetic nanostructures. Low-abundance miRNA-let-7a as a model can efficiently initiate the operation of the entropy-driven DNA amplifier, and the released output DNAs can open the partially hybridized double-stranded DNA anchored on Fe₃O₄@SiO₂ particles. The liberated Au nanoparticles (NPs)-cDNA can completely hybridize with CdSe/ZnS quantum dots (QDs)-cDNA-1 and result in proportionally decreased photocurrent of CdSe/ZnS QDs-cDNA-1. This unique entropy-driven amplification strategy is beneficial for reducing the reversibility of each step reaction, enables the base sequence invariant and the reaction efficiency improvement, and exhibits high thermal stability and specificity as well as flexible design. These features grant the PEC biosensor with ultrasensitivity and high selectivity. Also, instead of solid-liquid interface assembly for conventional EPI-based PEC biosensors, herein, DNA hybridization in the solution phase enables the improved hybridization efficiency and sensitivity. In addition, superparamagnetic Fe₃O₄@SiO₂ particles further ensure the enhancement of the selectivity and reliability of the as-designed PEC biosensor. Particularly, this single-step electrode modification procedure evidently improves the electrode fabrication efficiency, reproducibility, and stability.

Noble metal-molybdenum disulfide nanohybrids as dual fluorometric and colorimetric sensor for hepatitis B virus DNA detection

Hepatitis B virus (HBV) infection is one of the global healthcare burdens, and its early diagnosis is crucial for the prevention of HBV-induced chronic hepatitis, liver fibrosis, cirrhosis, and hepatocellular carcinoma. Although different detection approaches have been reported, most of these methods either rely on sophisticated machines or procedures, which limit their use particularly in the high endemic, developing countries. In this work, we report a dual-sensing nanoplatform built on noble metal-molybdenum disulfide (MoS₂) nanohybrids, and this platform can detect the HBV DNA target through either fluorometric or colorimetric readouts. The design with the silver nanocluster (AgNC)-MoS₂ nanohybrid enables multiplex fluorescent detection, while the HBV DNA-regulated growth of platinum nanoparticles (PtNPs) on the MoS₂ nanosheets offers signal-on colorimetric detection. Both AgNC-MoS₂ and PtNP-MoS₂ nanohybrids show high sensitivity with pico-molar detection limit and single nucleotide specificity, even with the spiked human serum. Collectively, the proposed nanohybrids possess their potential in the use of early HBV diagnosis, particularly suitable for the high endemic areas with limited medical and instrumental supports.

Hybridization chain reaction and its applications in biosensing

To pursue the sensitive and efficient detection of informative biomolecules for bioanalysis and disease diagnosis, a series of signal amplification techniques have been put forward. Among them, hybridization chain reaction (HCR) is an isothermal and enzyme-free process where the cascade reaction of hybridization events is initiated by a target analyte, yielding a long nicked dsDNA molecule analogous to alternating copolymers. Compared with conventional polymerase chain reaction (PCR) that can proceed only with the aid of polymerases and complicated thermal cycling, HCR has attracted increasing attention because it can occur under mild conditions without using enzymes. As a powerful signal amplification tool, HCR has been employed to construct various simple, sensitive and economic biosensors for detecting nucleic acids, small molecules, cells, and proteins. Moreover, HCR has also been applied to assemble complex nanostructures, some of which even act as the carriers to execute the targeted delivery of anticancer drugs. Recently, HCR has engendered tremendous progress in RNA imaging applications, which can not only achieve endogenous RNA imaging in living cells or even living animals but also implement imaging-guided photodynamic therapy, paving a promising path to promote the development of theranostics. In this review, we begin with the fundamentals of HCR and then focus on summarizing the recent advances in HCR-based biosensors for biosensing and RNA imaging strategies. Further, the challenges and future perspective of HCR-based signal amplification in biosensing and theranostic application are discussed.

A Dual Functional Self-Enhanced Electrochemiluminescent Nanohybrid for Label-Free MicroRNA Detection

The development of electrochemiluminescent (ECL) emitters with both intense ECL and excellent film-forming properties is highly desirable for biosensing applications. Herein, a facile one-pot preparation strategy was proposed for the synthesis of a self-enhanced ECL emitter by co-doping Ru(bpy)₃²⁺ and (diethylaminomethyl)triethoxysilane (DEAMTES) into an in situ-produced silica nanohybrid (DEAMTES@RuSiO₂). DEAMTES@RuSiO₂ not only possessed improved ECL properties but also exhibited outstanding film-forming ability, which are both critical for the construction of ECL biosensors. By coupling branched catalytic hairpin assembly with efficient signal amplification peculiarity, a label-free ECL biosensor was further constructed for the convenient and highly sensitive detection of miRNA-21. The as-fabricated ECL biosensor displayed a detection limit of 8.19 fM, much lower than those in previous reports for miRNA-21 and showed superior reliability for detecting miRNA-21-spiked human serum sample, demonstrating its potential for applications in miRNA-associated fundamental research and clinical diagnosis.

Complementary atomic flame/molecular colorimetry dual-mode assay for sensitive and wide-range detection of cancer cells

Since multicolor switch was involved in both Sr²⁺-related flame and precipitation reactions, these reactions can be thus utilized for constructing a complementary atomic flame/molecular colorimetry dual-mode sensing platform for a sensitive and wide-range analysis of cancer cells by virtue of the gas generation from platinum nanozyme-mediated hydrogen peroxide decomposition.

One-step enzyme-free detection of the miRNA let-7a via twin-stage signal amplification

MicroRNAs (miRNAs) play a significant role in diverse biological processes. The abnormal expression of miRNAs is related to the development of cancers and various diseases. It is of great importance to sensitively and accurately detect miRNAs for early disease diagnosis and treatment. Here, a new fluorescence strategy was initially proposed for the enzyme-free sensing of let-7a by combining the strand displacement reaction (SDR) with the hybridization chain reaction (HCR). The sensor was successfully applied to the detection of the let-7a gene with a wide linear range from 25 pM to 250 nM and a limit of detection (LOD) of 9.01 pM. The fluorescence intensity has a good linear relationship with the logarithm of the target concentration. In addition, the biosensor allowed for the highly sensitive detection of the target genes even in complex human serum samples. With simple operation yet improved detection capability for let-7a, the developed fluorescent biosensor thus shows great potential for early clinical diagnosis as well as biological research.

An enzyme-free fluorescent biosensor for highly sensitive detection of carcinoembryonic antigen based on aptamer-induced entropy-driven circuit

Carcinoembryonic antigen (CEA) is a disease biomarker, which can reflect the existence of tumors. The accurate detection of CEA in clinical samples is highly valuable for diagnosis of tumors. Herein, we developed an enzyme-free fluorescent biosensor for highly sensitive detection of CEA based on an aptamer-induced entropy-driven circuit. The aptamer hairpin specifically bound to CEA to expose the locked domain. Then, the exposed domain could trigger disassembly of multiple fluorophore strands from the three-strand complexes with the aid of fuel strands, leading to the production of remarkable amplified fluorescent signals. The one-step and homogeneous method exhibited high specificity and a wide linear range from 10 pg mL^-1 to 500 ng mL^-1 with a low limit of detection of 4.2 pg mL^-1. What's more, the whole detection process could be performed within 45 min and did not involve the use of any protein enzymes and antibodies. The developed strategy could also be applied to detect CEA in clinical samples with satisfactory results. Therefore, the strategy is an alternative sensing method for the detection of CEA.

Cooperative Metal Ion-Mediated Adsorption of Spherical Nucleic Acids with a Large Hysteresis

Spherical nucleic acids (SNA) refer to nanoparticles attached with a high density of oligonuleotides. Linear and spherical nucleic acids have many differences such as hybridization affinity, melting transition, and cellular uptake. In this work, these two types of DNA of the same sequence were compared for adsorption on polydopamine (PDA) nanoparticles and graphene oxide (GO). We focused on the effect of metal ions including Na⁺, Ca²⁺, and Zn²⁺ since metal ions are indispensible for DNA adsorption on PDA and GO. Gold nanoparticles (AuNPs) of various sizes were used to prepare the SNAs. For both PDA and GO, a normal binding curve of one metal ion was obtained for adsorbing the linear DNA, while the spherical DNAs larger than 5 nm showed a sigmoidal binding curve requiring multiple metal ions. Urea and EDTA were used to probe DNA adsorption affinity, where the spherical DNA showed stronger adsorption in general. In the presence of 300 mM Na⁺, 4 M urea or 4 mM EDTA failed to desorb the 13 nm spherical DNA. The spherical DNA showed a very large hysteresis of metal-dependent adsorption. This study demonstrates another unique property of SNA compared to linear DNA, revealing interesting orientation and packing of DNA on AuNPs, which has deepened our understanding of DNA interface chemistry.

The Recent Development of Hybridization Chain Reaction Strategies in Biosensors

With the continuous development of biosensors, researchers have focused increasing attention on various signal amplification strategies to pursue superior performance for more applications. In comparison with other signal amplification strategies, hybridization chain reaction (HCR) as a powerful signal amplification technique shows its certain charm owing to nonenzymatic and isothermal features. Recently, on the basis of conventional HCR, this technique has been developed and improved rapidly, and a variety of HCR-based biosensors with excellent performance have been reported. Herein, we present a systematic and critical review on the research progress of HCR in biosensors in the last five years, including the newly developed HCR strategies such as multibranched HCR, migration HCR, localized HCR, in situ HCR, netlike HCR, and so on, as well as the combination strategies of HCR with isothermal signal amplification techniques, nanomaterials, and functional DNA molecules. By illustrating some representative works, we also summarize the advantage and challenge of HCR in biosensors, and offer a deep discussion of the latest progress and future development trends of HCR in biosensors.

Assembly of "carrier free" enzymatic nano-reporters for improved ELISA

Enzyme-linked immunosorbent assay (ELISA) is an economic and easy operation technique that has been widely used for the detection of protein in industry. However, the low loading capacity of the enzyme reporter has contributed to the low sensitivity of traditional ELISA, and the cross-linking procedures of the enzyme-labeled antibody in ELISA methods can lead to the inactivation of the enzyme, which will further decrease the sensitivity. To address this issue, herein we fabricated "carrier-free" nanoparticles to obtain a horseradish peroxidase (HRP) labelled reporter with a high HRP loading capacity. A disulphide-containing bis-N-hydroxy succinimide (NHS) crosslinker (NHS-SS-NHS) was used to control the link and release of traceless HRPs, thus without reduction of its enzymatic activity. The HRP nanoparticle (NanoHRP) was successfully applied for dot blotting and ELISA. When carcinoembryonic antigen (CEA) was used as a target, the detection limit of the NanoHRP-based ELISA was 0.005 ng mL^-1, which was about 400 times more sensitive than traditional ELISA. A good correlation between the CEA concentrations and the response values measured by NanoHRP ELISA was obtained in the range of 0.005 to 1 ng mL^-1. This concept could be exploited to improve ELISA tests, especially those requiring a high accuracy, to facilitate physicians in deciding the appropriate medical treatment.